The present invention relates to the deactivation of microorganisms in a high-strength-electric field treatment system, and more particularly to the deactivation of microorganisms in such system wherein uniform flow of a product is effected. Even more particularly, the present invention relates to a treatment system for deactivating microorganisms in which uniform product flow is effected using electrodes with shapes selected to effect such uniform product flow.
As used herein the phrases "deactivating organisms," "deactivate organisms," "deactivation of organisms" and similar phrases refer to the killing or sterilization of living organisms such as bacteria, viruses, fungi, protozoa, parasites and the like.
Substantial technical effort has been directed to the preservation of perishable fluid food products such as milk products, natural fruit juices, liquid egg products, and pumpable meat products, such as ground beef or turkey. Such liquid food products may normally contain a wide variety of microorganisms, and are excellent culture media for such microorganisms, (i.e., are excellent bacteriological growth media).
Practical preservation methods which have found significant commercial application predominantly utilize heat treatment such as pasteurization to inactivate or reduce the microorganism population. For example, milk products are conventionally pasteurized at a minimum temperature of at least about 72.degree. C. for 15 seconds (or equivalent time/temperature relationship) to destroy pathogenic bacteria and most of the nonpathogenic organisms, with degradative enzyme systems also being partially or totally inactivated. However, products processed in this manner are still generally unsterile and have limited shelf life, even at refrigeration temperature.
The shelf life of liquid foodstuffs may be substantially extended by higher heat treatment processes such as "ultra high pasteurization", or "ultra high temperature" ("UHT") treatment. UHT treatment may be at a temperature of 140.degree. C. for four seconds. These processes are used in conjunction with aseptic packaging to achieve complete destruction of all bacteria and spores within the food product, however, such heat treatment typically adversely affects the flavor of the food product, at least partially denatures its protein content or otherwise adversely affects desired properties of the fluid food product.
Other approaches to liquid food preservation, which also have certain disadvantages, include the use of chemical additives or ionizing radiation.
The bactericidal effects of electric currents have also been investigated since the end of the 19th century, with various efforts having been made to utilize electrical currents for treating food products. Such efforts are described in U.S. Pat. Nos. 1,900,509, 2,428,328, 2,428,329 and 4,457,221 and German Patents 1,946,267 and 2,907,887, inter alia, all of which are incorporated herein by reference. The lethal effects of low-frequency alternating current with low electric field strength have been largely attributed to the formation of electrolytic chemical products from the application of current through direct contact electrodes, as well as ohmic heating produced by current flow through an electrically resistive medium. Unfortunately however, the electrolytic chemical products generated by low frequency, low strength electric field methods may be undesirable in fluid foodstuffs, and heating, as noted above, may also cause undesirable effects in the fluid foodstuffs.
As described in U.S. Pat. No. 3,594,115, incorporated herein by reference, lethal effects of high voltage arc discharges have been attributed to electrohydraulic shock waves. The utilization of explosive arc discharges to produce microbiologically lethal shock waves has not found wide-spread application as it is not a very effective means for preserving edible liquid foodstuffs. In addition, such explosive arc discharges can produce undesirable chemical byproducts in the foodstuffs being treated.
More recently, the effect of strong electric fields (or very high strength electric fields) on microorganisms has been studied as a mechanism for reversibly or irreversibly increasing the permeability of the cell membrane of microorganisms and individual cells. The application of very high strength electric fields to reversibly increase the permeability of cells has been used to carry out cell fusion of living cells and to introduce normally excluded components into living cells. Very high strength electric fields in nonnutrient media can also have a direct irreversible lethal effect upon microorganisms with the rate of deactivation dependent upon the field strength above a critical field level and the duration of the applied very high strength electric field.
A pulsed field treatment apparatus, which uses very high strength electric field pulses of very short duration, to deactivate microorganisms in food products is shown in U.S. Pat. Nos. 5,514,391 (the '391 patent); 5,235,905 (the '905 patent); and 5,048,404 (the '404 Patent), issued to Bushnell et al., and U.S. Pat. Nos. 4,838,154 (the '154 patent); and 4,695,472 (the '472 patent), issued to Dunn et al., all of which are incorporated herein by reference. The prevention of electrophoretic and electrochemical effects in these apparatuses is described in U.S. Pat. Nos. 5,393,541 and 5,447,733, issued to Bushnell, et al. (the '541 patent and the '733 patent), both of which are incorporated herein by reference. Generally, in accordance with the these patents, methods and apparatuses are provided for preserving fluid foodstuffs (or pumpable foodstuffs), which are normally excellent bacteriological growth media. Such preservation is achieved by applying very high strength electric field pulses (of at least about 5000 V/cm) of very short duration (of no more than about 100 microseconds) through all of the pumpable foodstuff.
By "pumpable," "liquid," or "fluid" "product" or "foodstuff" is meant a product, such as an edible, food product or other product, having a viscosity or extrusion capacity such that the product may be forced to flow through a treatment zone, e.g., a viscosity of less than about 1000 centipoise. These products include extrudable products, such as doughs or meat emulsions such as hamburger; fluid products such as beverages, gravies, sauces, soups, and fluid dairy products such as milk; food-particulate containing food slurries such as stews; food-particulate containing soups, and cooked or uncooked vegetable or grain slurries; and gelatinous foods such as eggs and gelatins; and other products, such as medical products, water and the like.
By "bacteriological growth medium" is meant that upon storage at a temperature in the range of 0.degree. C. to about 30.degree. C., the product, with its indigenous microbiological population or when seeded with test organisms, will demonstrate an increase in biological content or activity as a function of time as detectable by direct microscopic counts, colony forming units on appropriate secondary media, metabolic end product analyses, biological dry or wet weight or other qualitative or quantitative analytical methodology for monitoring increase in biological activity or content. For example, under such conditions the microbiological population of a pumpable foodstuff that is a bacteriological growth medium may at least double over a time period of two days.
The compositions of typical fluid foodstuffs that are biological growth media, derived from "Nutritive Value of American Foods in Common Units", Agriculture Handbook No. 456 of the U.S. Department of Agriculture (1975), are as follows:
______________________________________ FLUID FOODSTUFFS Fluid Carbo- Food Water Protein Fat hydrate Na K Product Wt % Wt % Wt % Wt % Wt % Wt % ______________________________________ Whole Milk 87.4 3.48 3.48 4.91 .05 .144 (3.5% fat) Yogurt ** 89.0 3.40 1.68 5.22 .050 .142 Raw Orange 88.3 .685 .20 10.0 .0008 .2 Juice Grape Juice 82.9 .OO1 tr. .166 .0019 .ll5 Raw Lemon 91.0 .41 .20 8.0 .0008 .14 Juice Raw Grape- 90.0 .48 .08 9.18 .0008 .16 fruit Juice Apple Juice 87.8 .08 tr. 11.9 .0008 .10 Raw Whole 73.7 12.88 11.50 .90 .12 .13 Eggs Fresh Egg 87.6 1O.88 .02 .79 .15 .14 Whites Split pea 70.7 6.99 2.60 16.99 .77 .22 Soup * Tomato 81.0 1.60 2.10 12.69 .79 .187 Soup * Tomato 68.6 2.0 .588 25.4 1.04 .362 Catsup Vegetable 91.9 2.08 .898 3.9 .427 .066 beef soup ______________________________________ * condensed commercial ** from partially skimmed milk
Very high strength electric fields may be applied by means of treatment cells of high-field-strength design, examples of which are described in detail by Bushnell et al. and Dunn et al. Basically, the foodstuff is, in practice, electrically interposed between a first electrode, and a second electrode. The very high strength electric field is generated between the first and second electrodes such that the very high strength electric field passes through the foodstuff, subjecting any microorganisms therein to the very high strength electric field. Generally, the second electrode consists of a grounded electrode, and a relatively higher or lower voltage potential is applied to the first electrode.
In the Bushnell et al. patents and the Dunn et al. patents, the pumpable fluid foodstuff is subjected to at least one very high strength electric field and current density electrical pulse, and at least a portion of the fluid foodstuff is subjected to a plurality of very high strength electric field and current density pulses, in a high-strength electric pulse treatment zone. In one processing technique, the liquid foodstuff is introduced into a treatment zone, or cell, between two coaxial electrodes that have a parallel configuration adapted to produce a substantially uniform electric field thereinbetween without dielectric tracking or other breakdown.
By "parallel" configuration it is meant that product passes between the electrodes, such that electric flux lines are approximately normal to direction of flow. Using these parallel-configured electrodes, very high strength electric field pulses are applied to the electrodes to subject the liquid foodstuff to multiple pulse treatment by the pulsed field apparatus. In order to generate the very high strength electric field pulses, the pulsed field apparatus employs, for example, a lumped transmission line circuit, a Blumlein transmission circuit and/or a capacitive discharge circuit. Alternatively, the Bushnell et al. patents describe the use of field reversal techniques in capacitive discharge systems (or pulse forming networks) to increase the effective potential across the treatment cell. For example, by applying a short electric field pulse of very high electric field strength (e.g., 20,000 volts per centimeter) across a treatment cell for a short period of time (e.g., 2 microseconds) of one polarity, followed by abrupt reversal of the applied potential within a short time period (e.g., 2 microseconds), an effective field approaching 40 kilovolts per centimeter is achieved across the cell.
If a product is continuously introduced into the treatment zone to which very high strength electric field pulses are periodically applied, and the product is concomitantly withdrawn from the treatment zone, the rate of passage of the product through the treatment zone can be coordinated with the pulse treatment rate so that all of the product is subjected to at least one very high strength electric field pulse within the treatment zone. The product may be subjected to treatment in a sequential plurality of such treatment zones, or cells, such as is described in more detail by Bushnell et al.
High-strength electric field treatment processes employ pulses of high-electric field strength to treat products as they pass between two electrodes. High voltage pulses applied to the electrodes expose the product to the high-strength electric fields. The high-strength electric fields in turn deactivate microorganisms carried by the product. The deactivation of microorganisms depends greatly on peak electric fields, pulse width and energy deposition in a given volume of the product as it flows between the electrodes. During flow between the electrodes, spatial variations in flow velocity with the product lead to variations in energy deposition between various volumes resulting in variations in deactivation affectivity in such volumes within the product being treated. This leads to reduced efficiency in the deactivation process in that over treatment of some volumes becomes necessary to assure a minimum energy deposition in all volumes. Prior art parallel-configured electrode treatment system designs all suffer from the problem of spatial variations in flow velocity and thus, exhibit variations in total energy deposition at various points (i.e., in various volumes) within the product being treated.
FIG. 1 shows one treatment system 100 in accordance with the prior art that exhibits spatial variations in flow velocity. As can be seen, a product enters the treatment system 100 radially through an inlet manifold 102 from one end of the treatment system (to the left as shown) and flows between each of two electrodes 104, 106 toward an outlet port 108 (to the right as shown).
Also shown in FIG. 1 are contour lines 110 showing interfaces between volumes of the product having different flow velocities. The contour lines show spatial variations in flow velocity.
As a result of the spatial variations in flow velocity, various eddy currents and variations in flow uniformity occur. These phenomenon result in non-uniform energy deposition within the product, and fouling of the treatment system, for example, "cooked" product that agglomerates on the electrodes.
FIG. 2 shows another treatment system 200 in accordance with the prior art in which a product flows essentially in an opposite direction (left to right) of that illustrated in FIG. 1. A small inlet pipe 202 (to the left) is a source of product flowing toward and between the electrodes 204, 206 with radial exit pipes in an exit manifold 208 (to the right) collecting the product following treatment.
As the product enters a treatment zone between the electrodes of FIG. 2, flow separation occurs as the product goes around a frustoconical "nose" 212 into the treatment zone between the electrodes 204, 206. This causes a nonuniform velocity distribution in the treatment zone. There is also flow separation that occurs as the product exits the treatment zone (as can be seen in the contour lines) leading to more eddys and more fouling of the treatment system.
Also shown in FIG. 2 are contour lines 210 showing interfaces between volumes of the product having different velocities. The contour lines illustrate the above-mentioned spatial variations in flow velocity.
As a result of the special variations in flow velocity illustrated in FIG. 2, eddy currents form and variations in flow uniformity occur causing fouling problems due to over-treated product and built up heat.
The present invention advantageously addresses the above and other needs.